In cellular telephone or other wireless voice or data communication systems, a service area is generally divided into cells, each of which may be further divided into sectors.
Each cell may be served by one or more base stations (BSs), which is further connected to a message switching center (“‘MSC’”), a subscriber management system (“SMS”), or a data router via a base station controller (“BSC”). A plurality of mobile communication devices/terminals (“MT”) are connected to the MSC, the SMS, or the router by establishing radio links with one or more nearby base stations.
One commonly used type of cellular radiotelephone communication system is referred to as a Code Division Multiple Access (CDMA) system, in which the radio signals of different users share the same frequency spectrum at the same time, in contrast to previous Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) systems.
In a typical CDMA cellular radiotelephone communication system, an MT communicates with a base station having the strongest available signal. In order to track the available signals, the MT maintains a list of available base stations. Specifically, each base station in the CDMA system transmits an unmodulated “pilot” signal at a set of predetermined frequencies. MT receives the pilot signals and determines which pilot signals are the strongest. A “searcher” unit located in the MT commonly performs the signal detection and intensity measurement functions. The results of the searcher are reported to the current base station. The current base station then instructs the MT to update a list of available base stations maintained by the MT. Typically, the list is further divided into three operative sets, an active set, a candidate set, and a neighbor set. The active set contains a list of the base stations with which the MT is currently communicating. The candidate set is a list of base stations which may change into the active set, and the neighbor set is a list of base stations which are being monitored, but less frequently.
Since the coverage area of a telecom system is traditionally divided into cells, an MT may move from one cell to another. As an MT moves and the signal from its currently active base station weakens, the MT must access a new base station. Based upon the results of the searcher and the instructions received back from the new base station, the MT will communicate with a different base station. The process of switching the communication link from one base station to another is known as a handoff process.
In order to transfer an MT user's communication without interruption, the communication link must be switched to the next base station. There are generally two types of handoff, a soft handoff and a hard handoff. If a new link is established before current link is terminated, it is known as the soft handoff. In contrary, in a hard handoff, the current link will be terminated first and then a new link with the new base station is established. Since the hard handoff will cause a service interruption for the MT user, although temporarily, it lowers the quality of service (QOS) for the cellular telephone user. For example, if the MT is engaged in a voice service, the user will most likely experience degraded voice quality or even call drop. If an MT is transmitting data, significant transmission delays (e.g., due to retransmission errors) will be likely to occur. The handoff usually occurs when an MT travels to a border area where base stations in multiple neighboring cells are available. When the MT is in the border area, signal interferences from multiple cells are severe, and the handoff procedure directly impacts communication quality.
An improved method and system for effectively performing a handoff procedure to achieve better communication quality would be desirable.